Defect-Engineered NaTb(MoO ₄ ) ₂ : Linking Structural Distortion to Enhanced Green Emission and Formaldehyde Sensing

The structural–property correlation in defect-engineered multifunctional oxides offers a promising strategy for tuning optoelectronic and sensing performance. Here, scheelite-type NaTb(MoO 4 ) 2 nanoparticles were synthesized via a controlled hydrothermal route to modulate the defect concentration in the system. Defect states have been utilized to engineer 5 D 4 → 7 F 5 transitions, which in consequence enhances green luminescence across the synthesized samples. Structural and X-ray photoelectron spectroscopy (XPS) analyses revealed that oxygen vacancy (V O )-induced disorder modulates the local electronic environment, leading to coexisting regular (Mo R O 4, Tb R O 8 ) and distorted (Mo D O 4, Tb D O 8 ) polyhedra. Density functional theory (DFT) calculations further demonstrated that V O -induced structural distortions promote electronic coupling between the TbO 8 and [MoO 4 ] 2– clusters. The optimized sample demonstrated the strongest charge-transfer band (CTB), attributed to enhanced charge transfer between Tb 3+ –O 2– and substantial charge accumulation around O atoms, as revealed by Bader charge analysis. The multifunctionality of NaTb(MoO 4 ) 2 was further evidenced by its excellent chemiresistive response toward toxic formaldehyde (HCHO) gas, exhibiting a high response ( S = 6.3 at 100 ppm) and ultrafast response/recovery times (4.8 s/32.2 s). The improved sensing performance is attributed to vacancy-mediated adsorption of O 2 – species and accelerated surface redox kinetics. This work elucidates the fundamental role of oxygen vacancies in tuning charge-transfer dynamics and provides a design pathway for rare-earth-based multifunctional materials for optoelectronic and environmental monitoring applications.